Semiconductor Die Collet and Method

ABSTRACT

Semiconductor device assembly die attach apparatus and methods are disclosed for improvements in attaching a semiconductor die to a die pad. Preferred methods of the invention include steps for positioning a semiconductor die on a bearing surface of a collet and retaining the die on the bearing surface of the collet using a vacuum force. A pushing force is also exerted on the die adjacent to the applied vacuum force. The pushing force opposes flexion of the die in the direction of the vacuum force. In further steps, the die is placed on a die pad, and die attach adhesive is interposed between the die and the die pad. A preferred method includes applying a pushing force to bow the central region of the die toward the die pad. In a preferred apparatus of the invention, a collet has a body including a bearing surface for receiving a die and a vacuum for holding it. A chamber encompassed by the bearing surface is adapted for applying the force of expelled gas against a die borne on the bearing surface. The collet is configured for holding a die surface against the bearing surface and for simultaneously pushing outward on the center region of the die so held.

This is a divisional of application Ser. No. 11/690,808 filed Mar. 24,2007, the contents of which are herein incorporated by reference in itsentirety.

TECHNICAL FIELD

The invention relates to electronic semiconductor devices andmanufacturing. More particularly, the invention relates to semiconductordevice manufacturing and packaging and to die handling apparatus andrelated methods.

BACKGROUND OF THE INVENTION

In conventional semiconductor device assembly, it is known to useadhesive to permanently mount a semiconductor die to a mounting pad orsubstrate. Typical adhesive die attach processes use curable adhesive,such as epoxy or polyimide, as die attach material to affix the die to adie pad, leadframe, substrate, or socket, for convenience referred toherein generally as the die pad. It is common in the art to dispense dieattach material in a controlled amount on a die pad. Die handlingequipment used for die attach processes typically employs apick-and-place tool to lift a die from a wafer tape or other holdingmechanism and place it on a die pad. The portion of the die handlingequipment that actually makes contact with the die is referred to as acollet. The die is placed on the collet, either by surface contact aloneor with assistance from a mechanical ejector pin guiding the die ontothe collet. A vacuum force exerted within the collet holds the die inthe collet while the tool moves it into the appropriate position forplacement on the pre-applied adhesive on the die pad.

Die handling presents technical challenges. Particularly for thin dice,which are becoming increasingly common in the arts, handling during dieattach requires great care to avoid cracks or other damage. Someexamples of die attach-related failure mechanisms known in the artinclude backside tool marks, scratches, or microcracks, which caneventually lead to die cracking. Thinner dice are in particular dangerfrom microcracks, which can result from excessive flexing of the dieduring handling. The contact surfaces of die attach collets aresometimes made from relatively soft plastic or elastomeric materialsinstead of metal in an effort to avoid causing mechanical damage on thedie surface. The practice of using a vacuum to hold the die in thecollet is another example of efforts to avoid inflicting damage tofragile dice. The use of a prior art vacuum collet, however, tends tocause a thin die to flex forming a concavity during die placement, whichcan cause further problems. Damage to the surface of the die can alsooccur, particularly in the central region, due to contact with thecollet.

The amount and distribution of die attach material between the die andthe die pad can be crucial to the secure attachment of the die to thedie pad and to the long term reliability of the completed assembly.Achieving the appropriate depth and uniform distribution of the dieattach adhesive layer, also called the bond line, is a significantchallenge. If the bond line is too thin, the bond may be insufficient tohold the die to the die pad. If the bond line is too thick, curing maybe inhibited or prolonged, the bond may tend to weaken over time,thermal performance may suffer, or other problems may result. An unevenbond line resulting from non-uniform adhesive distribution can result insimilar problems or in a combination of such problems. One of the mostthreatening problems a non-uniform distribution of adhesive can createis the formation of voids in the adhesive between the die and the diepad. Voids can lead to failures due to insufficient adhesive coverage orthermally induced stresses, for example. The formation of a concavity inthe die surface, caused by the flexing of a thin die placed in a vacuumcollet common in the arts, can induce these and other problems.

Generally, in addition to the formation of the bond line, a quantity ofadhesive is pressed from between the die and the bond pad during dieplacement. The formation of adhesive that builds from the bond pad tothe edges of the die is known as the die attach fillet. The formation ofthe fillet can be adversely affected by the excess, lack, or non-uniformdistribution of adhesive in the formation of the bond line. Excessivedie attach fillet may lead to die attach contamination of the diesurface. Too little fillet may reduce the strength of the attachment andlead to eventual problems such as die lifting or die cracking. Theformation of a proper fillet may be impeded by excessive, inconsistent,or unpredictable die flexion during placement of the die on the adhesiveby a die handling collet.

Due to these and other problems, it would be useful and advantageous toprovide semiconductor die handling apparatus and manufacturing methodswith improved die handling capabilities, particularly for use withrelatively thin and delicate dice.

SUMMARY OF THE INVENTION

In carrying out the principles of the present invention, using methodsand equipment compatible with established manufacturing processes,improved semiconductor die handling collets contribute useful advantagesto the art. The invention provides apparatus for the handling andplacement of semiconductor dice and superior die attach techniques,resulting in improved semiconductor device assemblies.

According to one aspect of the invention, a method for attaching asemiconductor die to a die pad includes steps for positioning a die on abearing surface of a collet and retaining the die on the bearing surfaceof the collet using a vacuum force. A pushing force is exerted on thedie adjacent to the applied vacuum force. The pushing force is used tooppose flexion of the die in the direction of the vacuum force. Infurther steps, the die is placed on the die pad, with die attachadhesive interposed between the die and the die pad, the die is thenreleased from the collet.

According to another aspect of the invention, in a method for attachinga semiconductor die to a die pad using a preferred embodiment of acollet, a pushing force is applied to the die in order to bow thecentral region of the die toward the die pad.

According to another aspect of the invention, a collet for handling asemiconductor die includes a body having a bearing surface for receivingthe die. A chamber in the body has an open side bounded by the bearingsurface, with a vacuum groove included in the bearing surface forholding a die against the bearing surface. A port is provided fortransmitting an expelled gas to the chamber. The parts are arranged sothat the vacuum force is adapted for holding the die surface against thebearing surface and the expelled gas is adapted for pushing the centerof the die out from the interior of the chamber.

According to yet another aspect of the invention, in a preferredembodiment, a collet is provided wherein an expelled gas may be appliedfor pushing outward on the center of a die held therein such that thedie extends outward beyond the plane of the bearing surface.

According to another aspect of the invention, a preferred embodiment ofa collet includes a flexible skin attached to the body of the collet andsituated for supporting a die held by the collet during handling.

According to still another aspect of the invention, a preferredembodiment of a semiconductor die attach system includes a collet havinga die bearing surface. The collet is configured for retaining the die onthe bearing surface using a vacuum force and also for applying a pushingforce to an adjacent portion of the die in opposition to inward flexionof the die in the direction of the vacuum force. The system alsoincludes a handling tool for moving the collet to a die pad, and placingthe die on the die pad.

According to another aspect of the invention, a collet system featuresan arrangement whereby a pushing force may be applied to cause outwardflexion of a die held by the collet such that a handling tool may bringthe center region of the die into contact with a die pad in advance ofthe periphery.

According to another aspect of the invention, a collet system includesprovisions for using pressurized gas for applying a pushing force to thecenter region of a die held to the collet by a vacuum force.

According to still another aspect of the invention, a semiconductordevice assembly of the invention includes a flexed die with a cured bondline formed with the central region of the die thinner than theperiphery.

The invention has advantages including but not limited to providingmethods, apparatus, and systems offering improvements in die handlingcapabilities useful in the manufacture of semiconductor device packages.These and other features, advantages, and benefits of the presentinvention can be understood by one of ordinary skill in the arts uponcareful consideration of the detailed description of representativeembodiments of the invention in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood from considerationof the following detailed description and drawings in which:

FIG. 1 is a bottom view of a die handling collet according to an exampleof a preferred embodiment of the invention;

FIG. 2A is a cutaway side view of the die handling collet according tothe example of a preferred embodiment of the invention of FIG. 1 shownin the context of a die attach system and method;

FIG. 2B is a cutaway side view of the die handling collet in acontinuation of the example of a preferred embodiment of the inventionof FIG. 2A shown in the context of a die attach system and method;

FIG. 2C is a cutaway side view of the die handling collet in acontinuation of the example of a preferred embodiment of the inventionof FIGS. 2A and 2B shown in the context of a die attach system andmethod;

FIG. 3 is a bottom view of a die handling collet according to an exampleof an alternative preferred embodiment of the invention;

FIG. 4A is a cutaway side view of the die handling collet according tothe example of a preferred embodiment of the invention of FIG. 3 shownin the context of a die attach system and method;

FIG. 4B is a cutaway side view of the die handling collet in acontinuation of the example of a preferred embodiment of the inventionof FIG. 4A shown in the context of a die attach system and method; and

FIG. 4C is a cutaway side view of the die handling collet in acontinuation of the example of a preferred embodiment of the inventionof FIGS. 4A and 4B shown in the context of a die attach system andmethod for implementing a preferred embodiment of a semiconductor deviceassembly of the invention.

References in the detailed description correspond to like references inthe various drawings unless otherwise noted. Descriptive and directionalterms used in the written description such as first, second, top,bottom, upper, lower, side, and so forth, refer to the drawingsthemselves as laid out on the paper and not to physical limitations ofthe invention unless specifically noted. The drawings are not to scale,and some features of embodiments shown and discussed are simplified oramplified for illustrating the principles, features, and advantages ofthe invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In general, the invention provides a die handling collet and relatedsystems and methods for improved die handling in semiconductor devicemanufacturing processes, particularly die attach processes. Referringprimarily to FIG. 1 and FIG. 2A, a bottom view (FIG. 1) and cutaway sideview (FIG. 2A) of a collet 10 according to a preferred embodiment of theinvention is described. A body 12, preferably made from plastic, metal,or other suitably rigid material, is capable of receiving a die 14 (FIG.2A). A vacuum groove 16 is provided at the edge of the body 12. Thevacuum groove 16 is incorporated into a die-bearing surface 18 of thebody 12, preferably entirely around the periphery. The vacuum groove 16is in communication with vacuum ports 20 for transmitting a vacuumforce, indicated by arrow 22, generated by a suitable mechanism such asa pump (not shown). The vacuum groove 16 preferably distributes thevacuum force around the periphery of the die-bearing surface 18 forholding a die 14 during handling and placement. A chamber 24 isincorporated within the body 12 and is preferably encompassed by thedie-bearing surface 18. One side of the chamber 24 is open such that adie 14 placed on the bearing surface 18 completes the enclosure. Withinthe chamber 24, a port 26 is provided for expelling pressurized gas,preferably air, indicated by arrow 28. The expelled gas 28 pressurizesthe chamber 24, exerting a pushing force on the adjacent surface of thedie 14. The pushing force preferably opposes the die flexion which tendsto occur due to the application of the vacuum force 22, preventing orreducing the temporary formation of a concavity on the outer surface ofthe die 14 due to flexing. Preferably, during die attach, the chamber 24is sufficiently pressurized to cause the die 14 to bow outward slightlyin a position convex to the adjacent die pad 32 or intervening dieattach adhesive 30 (FIG. 2A).

Now referring primarily to FIG. 2B, the collet 10 is shown in thecontext of further steps in a die attach method according to preferredembodiments of the invention. As illustrated, using the preferredembodiment of the collet 10 shown and described above, the expelled air28 within the chamber 24 is preferably used to flex the die 14 in orderto present a convex surface to the die attach material 30 pre-applied tothe die pad 32. The convex surface of the die 14, as indicated by arrows34, tends to expel air from between the die 14 and die attach adhesive30 during the ultimate placement of the die, reducing the frequency andmagnitude of void formation. Although the use of a convex surface ispreferred, in an alternative embodiment, the die may be flexed by theexpelled air 28 by an amount adapted to counter any inward flexioncaused by the vacuum 22, and calculated to prevent the outward flexionof the die 14. This implementation may be preferred for example withparticularly delicate dice, preventing or attenuating flexion andpresenting a substantially flat die surface to the die pad 32 and theintervening die attach adhesive 30, promoting a uniform thickness of dieattach material 30. It should be appreciated by those skilled in thearts that other alternative embodiments are possible without departurefrom the invention, for example, die attach processes using die adhesivefilm may also advantageously use the invention. As depicted in FIG. 2C,as the collet 10 brings the die 14 into position on the die attachadhesive 30, the pushing and vacuum forces may be reduced or eliminated,ultimately enabling the collet 10 to be removed after the die 14 isplaced.

An alternative preferred embodiment of a collet 40 of the invention isdepicted in a bottom view in FIG. 3, and in corresponding cutaway sideviews in FIG. 4A through 4C showing an example of a system and methodfor its use. As described elsewhere herein, the collet 40 has a body 12preferably made from plastic, metal, or like material and is capable ofreceiving a die 14 as shown. A vacuum groove 16 is incorporated into thedie-bearing surface 18 of the collet 40, preferably at the edge of thebody 12 and around its periphery. The vacuum groove 16 is provided witha vacuum force 22 through suitable vacuum ports 20. The vacuum groove 16preferably evenly distributes the vacuum force 22 around the peripheryof the die-bearing surface 18 for holding a die 14 during handling andplacement. An interior chamber 24 is incorporated within the die-bearingsurface 18 of the body 12. A port 26 is provided for expellingpressurized air or other gas 28 into the chamber 24. As in the otherpreferred embodiment described, the expelled air 28 pressurizes thechamber 24 to prevent the formation of a concavity in the outer surfaceof an adjacent die 14 due to flexion in response to the application ofthe vacuum force 22. Preferably, during die attach the chamber 24 issufficiently pressurized to cause the die to bow outward slightly in aposition convex to the adjacent die attach adhesive 30 as shown in FIG.4A and FIG. 4B. In this alternative embodiment, the collet 40 alsoincludes a support skin 42. The support skin 42 is preferablypermanently attached to the die bearing surface 18 of the collet body12. The support skin 42 is made from a flexible material such as, forexample, a thin film of Teflon, Mylar, (both registered trademarks ofDuPont Corporation), polymer, or the like. In operation, while thevacuum 22 exerted in the vacuum groove 16 holds the die 14, the pushingforce of air 28 expelled into the cavity 24 pressurizes the support skin42. As a result, the center region of the die 14 may be caused to bowoutward in a shape convex relative to the die attach adhesive 30. Aswith the above-described embodiments, using this preferred method, thebowed center region of the die 14 die contacts the die attach adhesive30 first and then spreads outward toward the periphery as it is movedtoward the die pad and as the pushing force on the die 14 is diminished.This sequence avoids the trapping of air during die attach, helps toform the die attach adhesive into a bond line 30 of uniform thickness,and fosters the formation of suitable fillets 36. Alternatively, forexample in cases where the die 14 may be particularly susceptible todamage from flexing, the outward pressure 28 may be regulated to holdthe die 14 substantially flat relative to the die pad 32 and attachadhesive 30. Thus the invention may be used for regulating the shape ofthe die surface presented to the die attach locale, balancing againstinward flexion exerted by the vacuum force 22, but refraining frombowing the die 14 outward in order to prevent inducing stress on the die14 in cases where increased gentleness is required. In anotheralternative embodiment, illustrated in the final position of the die 14in FIG. 4C, the bond line 30 is cured with a “smile” profile as shown,preferably uniformly thinner in the central region of the die 14 andprogressively thicker approaching the periphery.

As shown and described herein, preferred embodiments of the inventioncontribute one or more useful advantages to the art. The inventionprovides advantages including but not limited to improved die handlingcapabilities and reductions in damage during die attach inmicroelectronic semiconductor device package assembly. While theinvention has been described with reference to certain illustrativeembodiments, the methods, apparatus, and systems described are notintended to be construed in a limiting sense. Various modifications andcombinations of the illustrative embodiments as well as other advantagesand embodiments of the invention will be apparent to persons skilled inthe art upon reference to the description, drawings, and claims.

1. A collet comprising: a body having a bearing surface for receiving a semiconductor die and a chamber having an open side bounded by the bearing surface; a vacuum groove in the bearing surface configured for pulling a die surface against the bearing surface; a vacuum port for communicating a vacuum force to the vacuum groove; and an expelling port for communicating an expelled gas to the chamber; whereby the vacuum force is adapted for holding the die surface against the bearing surface and the expelled gas is adapted for asserting a force at the center region of a held die in a direction opposite the vacuum force.
 2. A collet according to claim 1 wherein the expelled gas is adapted for pushing the center region of a held die outward beyond the plane of the bearing surface.
 3. A collet according to claim 1 further comprising a flexible skin attached to the body and covering the chamber.
 4. A system comprising: a collet configured for retaining a semiconductor die on a bearing surface using a vacuum force applied to a portion of a first surface of the die, the collet also configured for applying a pushing force in a direction opposite the vacuum force to a portion of the first surface of the retained die.
 5. A system according to claim 4 whereby the pushing force causes outward flexion of the retained die held by the collet such that a handling tool may bring the outwardly flexed region of the second surface of the die into contact with the die pad before the periphery of the second surface of the die contacts the die pad.
 6. A system according to claim 4 further comprising means for applying the pushing force using pressurized gas.
 7. A system according to claim 4 further comprising a flexible skin attached to the bearing surface of the collet.
 8. A semiconductor device assembly comprising: a semiconductor die affixed to a die pad by a die attach adhesive, the die attach adhesive forming between the die and the die pad a film, which is thinner at the center of the die and thicker at the periphery of the die. 